Fuel cell systems are increasingly being used as a power source in a wide variety of applications. Fuel cell systems have been proposed for use in power consumers such as vehicles as a replacement for internal combustion engines, for example. Such a system is disclosed in commonly owned U.S. patent application Ser. No. 10/418,536, hereby incorporated herein by reference in its entirety. Fuel cell systems may also be used as stationary electric power plants in buildings and residences, as portable power in video cameras, computers, and the like. Typically, the fuel cell systems generate electricity used to charge batteries or to provide power for an electric motor.
Fuel cells are electrochemical devices which directly combine a fuel such as hydrogen and an oxidant such as oxygen to produce electricity. The oxygen is typically supplied by an air stream. The hydrogen and oxygen combine to result in the formation of water. Other fuels can be used such as natural gas, methanol, gasoline, and coal-derived synthetic fuels, for example.
The basic process employed by a fuel cell system is efficient, substantially pollution-free, quiet, free from moving parts (other than an air compressor, cooling fans, pumps and actuators), and may be constructed to leave only heat and water as by-products. The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells, depending upon the context in which it is used. The plurality of cells is typically bundled together and arranged to form a stack, with the plurality of cells commonly arranged in electrical series. Since single fuel cells can be assembled into stacks of varying sizes, systems can be designed to produce a desired energy output level providing flexibility of design for different applications.
Different fuel cell types can be provided such as phosphoric acid, alkaline, molten carbonate, solid oxide, and proton exchange membrane (PEM), for example. The basic components of a PEM-type fuel cell are two electrodes separated by a polymer membrane electrolyte. Each electrode is coated on one side with a thin catalyst layer. The electrodes, catalyst, and membrane together form a membrane electrode assembly (MEA).
In a typical PEM-type fuel cell, the MEA is sandwiched between “anode” and “cathode” diffusion media (hereinafter “DM's”) or diffusion layers that are formed from a resilient, conductive, and gas permeable material such as carbon fabric or paper. The DM's serve as the primary current collectors for the anode and cathode, as well as provide mechanical support for the MEA. Alternatively, the DM may contain the catalyst layer and be in contact with the membrane. The DM's and MEA are pressed between a pair of electrically conductive plates which serve as secondary current collectors for collecting the current from the primary current collectors. The plates conduct current between adjacent cells internally of the stack in the case of bipolar plates and conduct current externally of the stack in the case of monopolar plates at the end of the stack.
The secondary current collector plates each contain at least one active region that distributes the gaseous reactants over the major faces of the anode and cathode. These active regions, also known as flow fields, typically include a plurality of lands which engage the primary current collector and define a plurality of grooves or flow channels therebetween. The channels supply the hydrogen and the oxygen to the electrodes on either side of the PEM. In particular, the hydrogen flows through the channels to the anode where the catalyst promotes separation into protons and electrons. On the opposite side of the PEM, the oxygen flows through the channels to the cathode where the oxygen attracts the protons through the PEM. The electrons are captured as useful energy through an external circuit and are combined with the protons and oxygen to produce water vapor at the cathode side.
Electrical connections at either end of a fuel cell stack must accommodate the varying height of the fuel cell stack. This must be done while maintaining strict space requirements, keeping cost low to manufacturers, and maintaining the ability to carry high currents. Prior attempts to maintain these parameters have been accomplished by using sliding joints, flexible braided connectors, and cantilever style bus bars.
Flexible braided connectors have failed as a practical means to meet the needs of vehicle manufacturers. The braided connectors contain air space in the braids and do not maintain the space efficiency of a solid connector. Despite a capability of allowing sufficient movement of a fuel cell stack, braided connectors have proven too expensive to be feasible, while additionally failing to fall within the stringent space requirements.
Cantilever style bus bars are an inadequate option as well. The cantilever style bus bars maintain electrical contact by a biasing force within the connector. While a cost effective option, the cantilever style bus bars require a large space to accommodate the varying height of a fuel cell stack, limiting the use thereof in vehicle applications.
Sliding joint connectors, commonly known as fork plugs, have a blade and a fork that allow for a small amount of movement while maintaining an electrical contact between the blade and the fork. For large sliding joint connectors, multiple lap joints and fasteners, along with electrical grease to reduce resistance, may be required. Additionally, the blades that connect to the fork are selected according to the height of the fuel cell stack. If the stack height changes over time, the sliding joint connector may not provide adequate engagement of the fork and blade. The sliding joint connectors have proved a restrictive option for electrical connections at the end of a fuel cell stack due to a high cost, space requirements, and limited range of movement. Accordingly, sliding joint connectors are not a desired choice for vehicle manufacturers.
It would be desirable to provide a cost effective electrical connector that allows for sufficient movement between two points in a fuel cell stack while maintaining strict space requirements.